1. Field of the Invention
The present invention relates to a laser recording apparatus which is suitably applied to an electrophotographic color reproducing machine or a laser printer.
2. Description of the Prior Art
Some electrophotographic color reproducing machine uses a semiconductor laser as means for forming an electrostatic latent image on a photosensitive image retainer in accordance with image signals corresponding to an original document.
This is because, in case a laser beam scanner is used for color recording, electrostatic images can be easily formed with a displacement for color separated images so that a clear color image can be recorded.
FIG. 13 is a schematic view showing the structure of one example of a laser beam scanner to be used of an electrophotographic color copying machine of that type.
In FIG. 13, reference numeral 11 designates a drum-shaped image retainer which is formed on its surface with a photoconductive/photosensitive layer of selenium or the like for forming an electrostatic (latent) image corresponding to an optical image.
The laser beam scanner 30 is equipped with a semiconductor laser 31 which is optically modulated in accordance with modulation signals binary-coded from image informations.
The laser beam emanating from the laser 31 is incident upon a rotary polygon mirror scanner, i.e., a deflector 34 through a collimator lens 32 and a cylindrical lens 33.
The laser beam thus deflected by that deflector 34 illuminates the surface of the image retainer 11 through a focusing f-.theta. lens 35 and a cylindrical lens 36.
Thus, the laser beam is deflected by the deflector 34 to scan the surface of the image retainer 11 at a constant rate in a predetermined direction a to effect the image exposure.
Incidentally, reference numeral 39 designates a photosensor for receiving the laser beam reflected by a mirror 38 to generate an index signal indicating the scanning start of the laser beam so that image data of one line are written with reference to that index signal.
The deflector 34 can be exemplified by not only the rotary polygon mirror, as shown, but also a mechanical vibration mirror such as a galvanometer.
An example of the apparatus for scanning with an optical beam by using a mechanical vibration mirror is presented by the galvanometer mirror scanner apparatus, as is disclosed in Japanese Patent Laid-Open No. 60944/1979.
FIG. 14 presents one example of the galvanometer 50.
The galvanometer 50 is composed, as well known in the art, of a reflecting mirror 51, a drive coil 52 and a ligament (i.e., rotary hanging string) for mechanically connecting the reflecting mirror 51 and the drive coil 52. The drive coil 52 is arranged in a fixed external DC magnetic field.
If a drive current of a predetermined frequency is fed to the drive coil 52, a predetermined electromagnetic force acts upon the drive coil 52 to vibrate the reflecting mirror 51.
As a result, if the reflecting mirror 51 is illuminated with the optical signal modulated by the aforementioned image signals, these optical signals are deflected to reach the image retainer 11 so that the optical scanning similar to the above ones are conducted.
Here, in the optical scanning apparatus using the rotary polygon mirror as described above, the deflector invokes the following problems because the polygon mirror is attached to and is rotationally driven by the motor to deflect the laser beam:
(1) The rotary polygon mirror itself is enlarged to cause a bottleneck in reducing the size of the optical scanning apparatus;
(2) The rotational noises of the motor and the wind noises of the rotary polygon mirror grow harsh, while the motor is rotating, so that the noises and vibrations cannot be reduced;
(3) Since the bearing of the drive motor for the rotary polygon mirror of a smaller size is usually of ball type, it will wear after a long use to deteriorate the stability and reliability of the rotations; and
(4) Since the ball bearing motor has its driving rate is about 1 kHz in terms of frequency, it cannot stand the use for high-speed scanning.
In case, on the other hand, the bearing is exemplified by a wear-resisting bearing such as air bearing, the machining accuracy required for the shaft and bearing is very severe. If this requirement cannot be met, the shaft will be fixed due to dust. Another problem is that the deflector is large-sized and expensive for the practical use.
Moreover, the rotary polygon mirror may establish noise light in the optical system due to the optical scatter on its reflecting surface. The extent of this optical scatter depends upon the surface accuracy and the coating material of the reflecting surface. No matter whether the scatter extent is more or less, the noise light is established to illuminate the image retainer 11 thereby to adversely affect the final image.
This causes the fogging of the image and the reduction in reproducibility of a thin line. This raises an innegligible problem for the laser recording apparatus which is required to have a high quality and a high resolution as high as 12 to 24 dots/mm.
In case, on the contrary, the mechanical vibration mirror shown in FIG. 14 is used as the deflector, the reflecting mirror 51 and the drive coil 52 are fabricated separately, and the ligament 53 is attached so that the individual parts are enlarged in addition to the following drawbacks:
(1) Since the ligament is made of a metal, the mirror is difficult to turn over a wide range; and
(2) Because of the metallic material, moreover, the ligament will experience the metallic fatigue after a long use so that stable vibrations cannot be obtained.
In case, moreover, the ligament, the mirror and the frame supporting them are made of different materials, the accordingly different coefficients of linear expansion may make it difficult to support and vibrate the mirror stably if the ambient temperature and the environmental condition drastically change. This instability in the mirror support and vibrations will adversely affect the quality of the final image as in a laser beam printer or a facsimile demanding a high-speed scanning.
This is because the position of the beam spot to hit the image retainer 11 is not fixed as a result of the movement of the mirror during the beam scanning. As a result, a straight line may be curved, or the equal distance between lines may become irregular.
As has been described hereinbefore, the laser recording apparatus has a high production cost and a short lifetime because it uses the rotary polygon mirror or the mechanical vibration mirror as the deflector. As a result, a number of drawbacks such as reductions in the reliability of the recording apparatus and in the recording quality are invoked.
If a quartz deflector is used as the deflector, on the other hand, it receives a predetermined electromagnetic force, if its drive coil is supplied with an AC signal of a predetermined frequency, so that the reflecting mirror is vibrated at a predetermined angle with a predetermined frequency.
As a result, a laser beam is deflected if it is modulated for illumination by image signals so that an electrostatic image corresponding to the image informations can be formed on an image retainer by focusing the deflected laser beam on the image retainer.
Especially in case the quartz deflector molded integrally with the base is used as the deflector, as described above, it has a strength to endure a high torsional moment.
As a result, if the drive coil of this deflector is supplied with an AC signal for effecting vibrations in or in the vicinity of a frequency intrinsic to the deflector, the reflecting mirror of the deflector will be vibrated uniformly and stably at a desired rotational angle and in a predetermined frequency.
However, this structure is developed essentially for an electromagnetic oscillograph so that it generally has a small deflection angle and a small frequency.
It has therefore been accepted to be difficult that such deflector is applied to a small-sized high-speed laser printer.
After a variety of investigations, we have found that a printer having high stability, durability and image quality despite of possibility of high-speed scanning can be provided contrary to our expectations by using that deflector under specified conditions and with proper controls.